U.S. Pat. No. 5,346,389, U.S. Pat. No. 5,202,303, and U.S. Pat. No. 5,437,099 disclose catalyzers of an initially mentioned type, each of which comprises several catalytically active channels and several catalytically inactive channels. The known catalyzers are produced using zigzag-shaped corrugated or folded sheets that are layered by way of a helical winding or folding back and forth. The corrugations or folds then form the channels of the catalyzer. One side of the respective sheet is constructed catalytically active by way of a catalyzer coating. In this way, the layering creates the catalytically active channels and the catalytically inactive channels. It is hereby possible to arrange the catalyzer coating in strip form transversely to the main flow direction on the sheet, so that an uncoated strip is positioned in the main flow direction of the catalyzer between two coated strips. Inside the catalytically active channels, the conversion or combustion of the fuel/oxidant mixture takes place in the coated areas. In essence, no conversion or combustion of the mixture takes place in the uncoated areas or in the catalytically inactive channels, so that this part of the mixture flow can be used for removing heat, i.e., for the cooling of the catalyzer.
U.S. Pat. No. 4,154,568 discloses a catalyzer of a principally different construction that is provided with several monolith blocks arranged consecutively in the main flow direction. The monolith blocks contain channels that are all catalytically active and extend parallel to the main flow direction. The channels of a monolith block located downstream have a smaller flow cross-section than those of the monolith block located upstream. This is meant to achieve a complete combustion of the fuel/oxidant mixture inside the catalyst, while in the catalyzers of this class only part of the gas mixture is supposed to be burned.
The burning of lean natural gas/air mixtures, for example with λ=2, based on palladium or platinum catalyzer materials requires temperatures of approximately 500° C. For special catalyzer materials, the ignition temperature can be reduced to 450° C. or less. The combustion reaction is kinetically limited during ignition. However, after the ignition of the combustion reaction, an increase in the catalytic activity of the catalyzer results in very high temperatures that are unsuitable for a permanent operation of the catalyzer. Accordingly, only part of the mixture is burned in the known catalyzers. The remaining fuel is supposed to be converted downstream from the catalyzer, for example in a suitable combustion chamber, by way of a homogeneous combustion. If, however, the fuel/oxidant mixture already becomes too hot inside the catalyzer, the homogeneous combustion also may start there, inside the channels, destroying the catalyzer.
Because of the one-sided coating with catalyzer material and a corresponding stacking or layering of the sheets used to construct the catalyzer, a catalyzer construction can be achieved, in which approximately half of all channels are completely catalytically coated, while the other half of the channels are uncoated. This makes it possible to effectively reduce the temperature increase in the catalyzer since the combustion of the mixture in the catalyzer is limited to the catalytically active channels and therefore to approximately 50%. While therefore almost no fuel exits from the catalytically active channels, almost unchanged mixture flows from the catalytically inactive channels. This results in a high fluctuation of the fuel concentration at the catalyzer outlet. If a combustion of the remaining fuel occurs before the partial flows exiting from the catalytically active channels and from the catalytically inactive channels are completely mixed with each other, temperature peaks may occur in association with the undesired production of NOx. Furthermore, the thickness of the boundary layer along the channel length may increase so that the conversion of the mixture takes place only slowly.
In a catalyzer with catalytically active channels and catalytically inactive channels, the catalyzer temperature or the outlet temperature of the gas mixture can be adjusted so low that the catalyzer has an adequate stability. In order to be able to thermally stabilize a homogeneous combustion, such as is necessary, for example, for generating hot gases for the operation of a gas turbine in a power plant installation, downstream from the catalyzer, for example in a combustion chamber, relatively high temperatures are necessary.